For applications in microelectronics, high carrier mobilities are desirable. It has been found that electron mobility in strained Si/SiGe channels is significantly higher than that in bulk Si. For example, measured values of electron mobility in strained Si at room temperature are about 3000 cm2/Vs as opposed to 400 cm2V/s in bulk Si. Similarly, hole mobility in strained SiGe with high Ge concentration (60%˜80%) reaches up to 800 cm2V/s the value of which is about 5 times the hole mobility of 150 cm2V/s in bulk Si. The use of these materials in state-of-the-art Si devices is expected to result in much higher performances, higher operating speeds in particular. However, the underlying conducting substrate for MODFETs and HBTs or the interaction of the underlying substrate with active device region in CMOS are undesirable features which limit the full implementation of high speed devices. To resolve the problem, an insulating layer is proposed to isolate the SiGe device layer from the substrate. Therefore, there is a need for techniques capable of fabricating strained Si/SiGe on insulator materials.
There are two available techniques for making SiGe-On-Insulator (SGOI). One is via SIMOX as reported in a publication by T. Mizuno et al., entitled “High Performance Strained-Si p-MOSFETs on SiGe-on-Insulator Substrates Fabricated by SIMOX Technology,” IEDM, 99-934. However, this method has several limits because the oxygen implantation induces further damages in the relaxed SiGe layer in addition to the existing defects caused by lattice mismatch. And, the high temperature anneal (>1100° C.) needed to form oxide after the oxygen implantation is detrimental to the strained Si/SiGe layers since Ge tends to diffuse and agglomerate at temperatures above 600° C., this effect becomes more significant when Ge content is higher than 10%.
The second technique of making SiGe on insulator is via selective etching with the aid of an etch stop. In U.S. Pat. No. 5,906,951 by J. O. Chu and K. E. Ismail which issued in May 1999, a method of utilizing wafer bonding and backside wafer etching in KOH with a p++-doped SiGe etch-stop to transfer a layer of strained Si/SiGe on a SOI substrate was described. However, the etching selectivity of SiGe to p++-doped SiGe etch-stop in KOH decreases sharply as the doping level in the etch stop layer is below 1019/cm3, therefore, the strained Si/SiGe layer may also be subjected to KOH etching if etching could not stop uniformly at the p++ SiGe etch-stop layer due to variation of dopants in the p++ etch-stop layer. Furthermore, since the SiGe etch-stop layer is heavily doped with boron in the range from about 5×1019 to about 5×1020/cm3, there are chances of auto-doping of the strained Si/SiGe during thermal treatment.
For fiber optic applications, SiGe/Si heterojunction diodes are a good choice for demodulating 1.3-1.6 um light at 300K. The use of 30% to 50% Ge is suggested to achieve absorption at the desired 1.3-1.6 um wavelength and low defects such as dislocations in the SiGe layer is needed to enhance the photodetector sensitivity. The state-of-the-art technology to achieve SiGe/Si heterojunction diodes with high responsitivity, low noise, and fast response is to form a 100-period SiGe/Si strained layer superlattice. However, the alloy then no longer behaves like the bulk material due to the quantum size effect. The net result of the quantum size effect is that the absorption occurs at wavelengths (1.1-1.3 um) shorter than expected. Therefore, a bulk SiGe alloy with desirable Ge content and low defects is needed to fabricate photodetectors that would absorb lights in the range of 1.3-1.6 um.
The invention provides a method capable of transferring a low defect SiGe layer onto a desirable substrate using the etch-back method but without any additional heavily doped etch-stop layer. The key feature of this invention is that a SiGe layer serves both as the layer over which the epitaxial strained Si/SiGe is grown but also as an etch-stop layer itself in some specific etching solutions. In other words, the SiGe layer is a self-etch-stop in this case. As a result, the process of fabricating strained Si/SiGe on insulator or a SiGe/Si heterostructure is greatly simplified and the quality of the strained Si/SiGe or SiGe/Si heterostructure is significantly improved.